The nature of quantum things

It is now well over a century since quantum theory was born with the seminal work of Planck on blackbody radiation. It took only a quarter of a century for quantum theory to develop a consistent formalism and show an amazing power of explanation and prediction. Yet, strange as it may seem, the conceptual status of fundamental quantum notions is still under dispute, despite decades of arguments. Most elementary textbooks and popularisation works on quantum physics remain plagued by archaic wordings and formulations. It seems as if the time is now ripe for re-examining the way quantum theory should be presented. In the current talk, I intend to contribute to this endeavour, by concentrating on the concepts used for describing the basic entities of quantum theory, while stressing the conceptual and terminological questions they raise.